msc: Add and use gsm_subscriber_group

Currently every subcriber object directly refers to the gsm_network
which contains a flag shared by every related subscriber
(keep_subscr). This adds a dependency on gsm_network even if only the
function defined in gsm_subscriber_base.c are used.

This patch adds a new struct gsm_subscriber_group which contains the
keep_subscr flag and a back reference to the network object. The
latter is not dereferenced in gsm_subscriber_base.c, so it can safely
be set to NULL when only that part of the gsm_subscriber API is being
used. It also changes that API to use gsm_subscriber_group instead of
gsm_network parameters.

Since there are some places where a pointer to the gsm_network is
needed but where only a gsm_subscriber is available, a 'net' back
pointer is added to the group struct, too. Nevertheless subscr group
and network could be separated completely, but this is not the topic
of this commit.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/db/db_test.c b/openbsc/tests/db/db_test.c
index 75c92d3..62fbb73 100644
--- a/openbsc/tests/db/db_test.c
+++ b/openbsc/tests/db/db_test.c
@@ -31,9 +31,10 @@
 #include <inttypes.h>
 
 static struct gsm_network dummy_net;
+static struct gsm_subscriber_group dummy_sgrp;
 
 #define SUBSCR_PUT(sub) \
-	sub->net = &dummy_net;	\
+	sub->group = &dummy_sgrp;	\
 	subscr_put(sub);
 
 #define COMPARE(original, copy) \
@@ -70,7 +71,7 @@
 	struct gsm_subscriber *subscr;
 	subscr = db_get_subscriber(GSM_SUBSCRIBER_IMSI, "9993245423445");
 	OSMO_ASSERT(subscr);
-	subscr->net = &dummy_net;
+	subscr->group = &dummy_sgrp;
 
 	sms = sms_alloc();
 	sms->receiver = subscr_get(subscr);
@@ -130,7 +131,7 @@
 		0xd0, 0xf1, 0xfd, 0x06, 0x00 };
 
 	rcv_subscr = db_get_subscriber(GSM_SUBSCRIBER_IMSI, "901010000001111");
-	rcv_subscr->net = &dummy_net;
+	rcv_subscr->group = &dummy_sgrp;
 
 	sms = db_sms_get(&dummy_net, 1);
 	OSMO_ASSERT(sms->id == 1);
@@ -159,6 +160,9 @@
 	osmo_init_logging(&log_info);
 	log_set_print_filename(osmo_stderr_target, 0);
 
+	dummy_net.subscr_group = &dummy_sgrp;
+	dummy_sgrp.net         = &dummy_net;
+
 	if (db_init("hlr.sqlite3")) {
 		printf("DB: Failed to init database. Please check the option settings.\n");
 		return 1;